U.S. Pat. No. 4,569,412 issued Feb. 11, 1986, to John V. Bouyoucos et al. describes a down hole seismic source. In that patent, the signal is generated by a hammer that is hydraulically driven and impacts on a piston assembly which is prebiased against the side of the hole.
U.S. Pat. No. 4,715,470 issued Dec. 29, 1987, to Bjorn N. P. Paulsson. This patent describes an electromagnetic driven seismic source. The downhole tool is secured to the wall of the well bore and a reaction mass is driven to impart shear waves into the surrounding formation. (See also U.S. Pat. No. 4,702,343 issued Oct. 27, 1987 to Paulsson).
In U.S. Pat. 4,648,478 issued Mar. 10, 1987, to Pascal Dedole et al., a large mobile mass is utilized to engage a target element within the apparatus to create a shock which is transferred into the well bore wall through elongated shoes which are pressed against the well bore wall by hydraulic cylinders.
U.S Pat. No. 4,747,466 issued May 31, 1988, to Bill L. Jaworski. This patent describes apparatus wherein a mass is accelerated using force generated by the surrounding high pressure liquid in the well bore. The seismic signal is generated by moving the mass into a dashpot arrangement which creates a controlled deceleration, and thus a shock wave is generated.
It is highly desirable that a downhole seismic source be capable of operation in cased wells as well as in open holes, that is, in noncased well bores. Also, it is highly desirable that such apparatus be capable of being used in shallow or deep well bores.